1. Field of the Invention
This invention relates to an idle speed control system for an engine which causes an idle regulator valve to control the amount of intake air to be fed to the engine when the throttle valve is closed so that the actual engine speed during idle converges on a target engine speed.
2. Description of the Prior Art
In recent electronic control engines, there has been in wide use the following idle speed control system as disclosed, for instance, in Japanese Unexamined Patent Publication No. 62(1987)-32239.
As shown in FIG. 9, an air cleaner 6, an airflow sensor 8, a throttle valve 10, an injector 12 are provided in an intake system 4 of an engine 2. A throttle position sensor 14 detects the opening of the throttle valve 10 and an idle switch 16 detects full closure of the throttle valve 10. A bypass passage 18 bypasses the throttle valve 10 and connects upstream and downstream sides of the throttle valve 10. An idle regulator valve (a solenoid valve) 20 is provided in the bypass passage 18.
Various sensors for detecting the operating condition of the engine 2 and the engine load condition, e.g., an intake air temperature sensor 22, an engine coolant temperature sensor 24, an engine speed sensor 26 and an air-fuel ratio sensor 28, are connected to a control unit 30. Though not shown, a compressor of an air conditioner, an oil pump of a power steering system and other auxiliary mechanisms are connected to the output shaft of the engine 2. In order to detect external load acting on the engine in response to driving of such auxiliary mechanisms, an air conditioner switch 32, a power steering switch 34 and the like are connected to the control unit 30.
The control unit 30 controls the engine 2 on the basis of information input from the sensors and switches.
The idle switch 16 is turned on when the throttle valve 10 is full closed. When the idle switch 16 is turned on, the control unit 30 determines a target idle speed No according to information on the operating condition of the engine such as the temperature of the engine coolant, whether external load is acting on the engine and the like, and calculates a basic mass flow of intake air required to maintain the target idle speed No. The control unit 30 corrects the basic mass flow according to the difference between the target idle speed No and the actual engine speed Ne, thereby obtaining a present target mass flow of intake air, and controls the opening of the idle regulator valve 20 on the basis of the target mass flow. After the next and later runs, so long as the target idle speed is not changed, the control unit 30 corrects the preceding target mass flow according to the target idle speed No and a newly detected actual engine speed Ne, thereby calculating a new target mass flow. In this way, the control unit 30 causes the difference between the target idle speed and the actual engine speed to converge on 0.
The idle regulator valve 20 is opened and closed by pulse signals of a sufficiently high predetermined frequency, and the effective opening degree of the idle regulator valve 20 is changed by changing the duty ratio of the pulse signals.
Generally, the engine speed is determined by the balance between the engine output torque and the load torque, and when the former is smaller than the latter, the engine speed is lowered. This will be described with reference to FIG. 10, hereinbelow.
In FIG. 10, line b represents the engine output torque (in terms of the air charging efficiency Cet1) required to operate the engine 2 at a given fixed speed. When the relation between the air charging efficiency and the engine speed is on the line b, the engine output torque conforms to the load torque and the engine speed is fixed.
The air charging efficiency Cetno when the engine 2 is fixedly operated at the target idle speed No with the mass flow of intake air kept at a value Gno required to fixedly operate the engine 2 at the target idle speed No is represented by the following formula (1). EQU Cetno=K.multidot.(Gno/No) (1)
Wherein K represents a mass flow-charging efficiency conversion coefficient.
Further, the air charging efficiency Cetne when the engine 2 is fixedly operated at a speed Ne with the mass flow of intake air kept at a value Gno required to fixedly operate the engine 2 at the target idle speed No is represented by the following formula (2). EQU Cetne=K.multidot.(Gno/Ne) (2)
The following formula (3) is derived from formulae (1) and (2). EQU Cetne=Cetno.times.(No/Ne) (3)
Line a in FIG. 10 represents formula (3).
When the opening of the idle regulator valve 20 is adjusted so that the mass flow of intake air is kept at a value Gno required to maintain the target idle speed No and the engine 2 is fixedly operated at a speed of Ne1by motoring, the air charging efficiency Cetne fed to the cylinder 2a of the engine 2 corresponds to the value for point A on the line a.
Since the air charging efficiency Cet1 required to maintain the engine speed Ne1 corresponds to the value for point A' on the line b, when motoring is interrupted in this state, a torque difference T1=Kt(Cet1-Cetne) (Kt being a coefficient) which corresponds to the difference between the air charging efficiency Cet1 for point A' and the air charging efficiency Cetne for point A is produced and the engine 2 begins to decelerate. When it assumed that the actual air charging efficiency moves along the line a as the engine speed Ne lowers, the torque difference T1 is nullified when the engine speed Ne is equalized to the target idle speed No. At this time, the engine output torque and the load torque balance with each other and the engine 2 begins to fixedly operate at the speed No.
However, as is well known, in a transient state of the operating condition of the engine where the engine speed Ne changes even if the air mass flow is fixed, the actual air charging efficiency Cetned (a first-order lag air charging efficiency) changes every stroke cycle of the engine 2 in the manner represented by the following formula. EQU Cetned(i)=KSKCCA.multidot.Cetned(i-1)+(1-KSKCCA).multidot.Cetne(i)(4)
wherein KSKCCA is a first-order lag coefficient.
Line c in FIG. 10 represents formula (4). As can be understood from line c, the torque difference T1 is larger than 0 at the time (point B) the engine speed Ne is equalized to the target idle speed No, and accordingly, the engine 2 further decelerates. Deceleration of engine 2 stops at the time (point C) Cetned becomes equal to Cet1. On the other hand, Cetned tends further increase and accordingly, the engine 2 comes to accelerate and finally the engine speed Ne converges on the target idle speed No. The graph shown in FIG. 11 shows such behavior of the engine speed.
When fuel feed is cut until the engine speed falls to a predetermined speed Ne2 during deceleration of the engine 2 as is commonly carried out, the engine output torque becomes 0 and accordingly the rate of deceleration increases. Further, when the engine 2 operates under external load such as the air conditioner, the power steering system and the torque convertor, the engine speed falls much more.
In the way described above, the engine speed falls when the engine speed is caused to converge on the target idle speed No during deceleration, and the engine speed falls because the first-order lag air charging efficiency Cetned at the time (point B) the engine speed Ne is transiently equalized to the target idle speed No during deceleration is short of the air charging efficiency Cetno which can balance with the engine load.
In order to overcome this problem, conventionally, the air mass flow is temporarily increased when deceleration of the engine is detected and thereafter gradually returned to the original value. However, this method is just like a symptomatic treatment and requires very large data for each of engines of different specifications in order to conform it all the operating conditions of the engine. Further, it requires a very complicated control program and experience to get matching.
Further, recently, there has been a trend toward enlargement of the volume of the intake passage downstream of the throttle valve, which leads to increase in the time lag before the air the flow rate of which is controlled by the idle regulator valve 20 is actually enters the cylinder, thereby causing the engine speed to fall more.